1. Field of the Invention
The present invention relates to polyurethane compositions which are soluble in organic solvents and harden to form insoluble coatings through the reaction of isocyanate groups.
2. Description of the Prior Art
Polyurethanes which cure through the reaction of terminal isocyanate groups are known. For example, U.S. Pat. No. 4,118,411 discloses diisocyanates based on the reaction product of glycols with diphenylmethane diisocyanate isomers in specific ratios. The diisocyanate products are suitable for the production of elastomers or foams.
Polyurethanes having terminal isocyanate groups which cure by reaction with moisture are also known. British Pat. No. 1,294,017 discloses the use of polyurethanes as binders in synthetic resin concretes. The active hydrogen component of the polyurethane is based on polyether polyols while the isocyanate component is based on toluene diisocyanate isomers or the isomers and higher homologues produced by the phosgenation of aniline-formaldehyde condensates. Note particularly Example 2.
U.S. Pat. No. 4,292,350 discloses polyurethane compositions based on specific monoisocyanates and polyisocyanates which contain an average of more than two isocyanate groups per molecule. These compositions may be applied by spraying with subsequent curing by moisture.
U.S. Pat. No. 3,792,023 is directed to one-component isocyanate-terminated polyurethanes prepared from aromatic diisocyanates and polyols, one of the polyols containing a tertiary nitrogen atom. These polyurethanes are not only stable in storage, but also dry quickly into coatings when exposed to atmospheric moisture through reaction of their terminal isocyanate groups. While these systems have found acceptance by virtue of this surprising combination of properties, they still suffer from several disadvantages.
Initially, it is difficult to formulate these polyurethanes in solvent blends which are not regulated by the government, e.g. California's Rule 66, without a reduction in properties. Secondly, these polyurethane systems cannot be formulated at high solids levels, and finally, coatings prepared from these systems must be applied in thin layers to prevent entrapment of bubbles which are formed from the reaction of the isocyanate groups with moisture. The bubbles are trapped when the surface of the coating reacts to form a skin which prevents the bubbles formed beneath the surface from escaping.
Accordingly, it is an object of the present invention to provide storage stable polyurethane compositions which can be formulated in nonregulated solvents at high solids contents.
It is an additional object to provide polyurethanes whose terminal isocyanate groups react with moisture to form coatings without the entrapment of bubbles.
It is a final object of the present invention to provide one-component polyurethanes which are compatible with pigments and also meet the above objectives.
It has now been found that these objectives can be achieved with a composition which contains isocyanate-terminated polyurethanes based on specific products of the aniline-formaldehyde condensation reaction and, optionally, specific monoisocyanates.